When you leave a political party to which you have remained faithful for the whole of your adult life, and set out to form a new grouping, you have no idea what is going to happen.

Many observers of the departure of seven MPs from the Labour Party say that because the Social Democratic Party, founded in March 1981, failed to win the 1983 general election, the new grouping must be doomed to failure.

And it is certainly true that under our electoral system, new parties find it very difficult to establish themselves. The last to do so at Westminster is Labour, its rise assisted by the split from 1916 in the Liberals.

But that is not quite the end of the argument. In recent years, UKIP has failed to establish itself as a party of government, but it did force the Conservatives to promise a referendum on EU membership. The No vote in that referendum has destabilised both main parties, and may well have created the conditions for a major realignment.

And whether or not such a realignment takes place, the SDP deserves a subtler verdict than outright failure. Labour survived because it adopted many of the SDP’s policies. The 1983 Labour manifesto was from the point of view of the SDP intolerable, but the 1997 manifesto on which Tony Blair led Labour (rebranded as “New Labour”) back into power was in many respects a tribute to the SDP.

The success of Jeremy Corbyn and his friends can in turn be seen as a kind of belated revenge by the Labour Left on Blair. After decades of being marginalised, the Left has seized control of the party.

Its domination has led to the present rebellion. The seven MPs who lead it – Chris Leslie, Luciana Berger, Ann Coffey, Chuka Umunna, Mike Gapes, Angela Smith and Gavin Shuker – are evidently not such heavyweight figures as the Gang of Four – Roy Jenkins, Shirley Williams, David Owen and Bill Rodgers – who founded the SDP.

But a rebellion of this kind can benefit from being underestimated. The unremarkable nature of the seven MPs may lead Corbyn and his friends to tell each other that no change of course is needed.

And a movement of this kind does not only depend on its leaders. One might even say that the SDP was hindered by having too many leaders.

What matters even more is the volume and enthusiasm of the followers. The creation of the SDP revealed the existence of a large number of people who were totally fed up with the existing parties, but were prepared to throw themselves body and soul into a new movement.

When Rodgers was wondering in the summer of 1980 whether to break with Labour, he records in his memoir, Fourth Among Equals, that

“David Marquand…urged me to make the break even if only three or four MPs were to follow. By staying, I might, he said, be able to keep the Labour Party from total self-destruction but I would not save it. The most I could achieve was ‘a ten-year (or 20-year) labour of Sisyphus, endlessly pushing the boulder up the hill only to see it roll down again’. It was a convincing image given the legitimate left’s continued tolerance of the wreckers, and the lack of stomach for the fight of Hattersley and others like him.”

In January 1981, when Rodgers, along with Jenkins, Williams and Owen, signed the Limehouse Declaration, in which they declared their intention to “rally all those who are committed to the values, principles and policies of social democracy”, and added that “the realignment of British politics must now be faced”, they could not tell what would happen:

“We knew that eight or nine other MPs would immediately join us and believed that we would soon get 100 names from amongst the great and the good to endorse our Council for Social Democracy. But otherwise we were in the dark about the response we would provoke, expecting to build steadily over a period of months to the launch of a new party. But the publicity given to the Limehouse Declaration brought a snowstorm of letters, which became an avalanche when the names of the first signatories to our Declaration for Social Democracy appeared in The Guardian on 5th February. I had letters from old school friends, former civil servants and, more predictably, men and women who had supported the Campaign for Democratic Socialism 20 years before. Instead of having to recruit, like Garibaldi, a thousand political irregulars with whom to start our bold campaign, we found that we had placed ourselves in the leadership of an army already formed and waiting…we decided to bring forward the launch to 26th March.”

Such things are inherently unpredictable. So are the changes and chances which may be offered in by-elections. When the Warrington by-election came up, Jenkins stood for the SDP, and astonished almost everyone by finishing in a strong second place, with 42 per cent of the vote.

Had Williams been the SDP candidate in Warrington, she might well have won, and become the new party’s leader, with a wider appeal than Jenkins. She was instead returned to Parliament at the Crosby by-election in November 1981, with Jenkins following at Glasgow Hillhead in March 1982.

Those were famous victories, which few would have predicted when the Limehouse Declaration was signed. Disappointment followed when at the general election of 1983, the SDP/Liberal Alliance gained 25.4 per cent of the vote, but only 23 seats, of which a mere six went to the SDP, while Labour, with 27.6 per cent, had 209 seats.

But although the obvious lesson of this is that the first-past-the-post electoral system favours the established parties, that law should not be regarded as immutable.

There comes a tipping point, not quite attained by the SDP in 1983, at which an insurgent party finds that first-past-the-post works in its favour. In the 2015 general election in Scotland, the SNP won 56 seats, compared to six in 2010, while Labour won a single seat, compared to 41 in 2010.

Our politics can be astonishingly volatile. If Corbyn and his advisers treat what happened yesterday as insignificant, it is all the more likely to turn out to be the start of something big.